In optical disc apparatus adapted to record information signals on or reproduce information signals from an optical disc that operates as information recording medium, a laser beam emitted from a laser beam source is converged by an objective lens and irradiated onto the signal recording layer of the optical disc to record or reproduce signals. When a laser beam is irradiated by way of an objective lens, spherical aberration appears depending on the thickness of the cover layer covering the recording layer of the optical disc, or the distance for the laser beam striking the optical disc to travel from the recording surface to the recording layer. In other words, the optical system of an optical disc apparatus that include an objective lens is designed to minimize the spherical aberration as a function of the thickness of the cover layer arranged on the optical disc to be used with it and hence spherical aberration takes place when the thickness of the cover layer differs from the expected value.
For the same token, it is also known that spherical aberration appears when the thickness of the cover layer arranged on a optical disc is not uniform. Additionally, in recent years, optical discs having a multilayered recording layer have been marketed to realize a high recording density. With such an optical disc, the distance from the laser beam striking surface to the recording layer varies so that spherical aberration takes place when signals are recorded on or reproduced from any of the recording layers other than the reference recording layer. When spherical aberration exists, the image forming performance of the laser beam irradiated on the signal recording layer is degraded to by turn degrade the information signal recording/reproducing characteristics. Therefore, means are required to correct the spherical aberration attributable to the thickness of the cover layer arranged on the optical disc.
Known means for correcting the spherical aberration attributable to the varying thickness of the cover layer provided on such an optical disc include those for deforming the surface profile of the mirror of the optical system. For example, Jpn. Pat. Appln. Laid-Open Publication No. 5-15191 discloses a deformable mirror including a deformable plate carrying a mirror surface on the surface thereof and piezoelectric actuators for pressurizing the deformable plate at several positions from the rear side of the plate so that the spherical aberration that appears is corrected by changing the voltages being applied to the respective piezoelectric actuators and thereby deforming the mirror surface.
Jpn. Pat. Appln. Laid-Open Publication No. 9-152505 also discloses a deformable mirror where a reference surface having a predetermined profile is formed under a flexible member carrying a mirror surface on the surface thereof and the flexible member is adapted to be adsorbed to the reference surface or de-adsorbed in order to produce two desired profiles. In other words, the reference surface is so profiled that the spherical aberration can be corrected by the profile of the mirror surface when the flexible member is adsorbed to the reference surface.
Of the above-cited arrangements, that of providing piezoelectric actuators is complex because of the necessity of using a plurality of piezoelectric actuators in order to make the mirror surface show a predetermined profile. Then, the control circuit for controlling the operation of driving the plurality of piezoelectric actuators inevitably shows large dimensions and makes the overall arrangement a very complex one.
Additionally, when recording information signals on and reproducing information signals from an optical disc developed in recent years for high density recording the beam diameter of the laser beam converged to the signal recording surface of such an optical disc is typically about 4 mm. Then, it is highly difficult to arrange a plurality of piezoelectric actuators in an area of such a dimension.
With the deformable mirror provided with a reference surface as cited above, it is possible to downsize the control circuit because it does not require the use of a plurality of piezoelectric actuators to realize a predetermined profile for the mirror surface. However, the mirror surface is deformed only in two modes of adsorbing and de-adsorbing the flexible member. With an optical system using such a deformable mirror, it is not possible to effectively correct the spherical aberration of an optical disc having three or more than three signal recording layers.